A big advantage of taking the time to read a book, even if the book has just one central idea, is that it forces you to spend 5+ hours thinking on that idea. No summary can do that for you. I'd even go so far as to say that it's not even important which book or idea you spend that time on; just spending time with your own thoughts and imagination will teach you things no non-fiction or summary can.

I do agree that summaries and comments are useful, but they serve the same purpose as non-fiction rather than fiction.